1. Field of the Invention
The present invention relates to an optical multiplexing and demultiplexing device of diffraction grating type which is used in a wavelength division multiplexing optical communication system.
2. Description of the Background Art
Due to the superior properties of the optical signal transmission concerning the transmission speed, the signal interference, etc. in comparison to the electric signal transmission, the optical communication techniques have been studied very actively, and in recent years, the wavelength division multiplexing optical transmission for handling the multi-channel wavelength division multiplexed optical signals has been attracting attentions.
Namely, the wavelength division multiplexing optical transmission is a transmission scheme suitable for the long distance, large capacity data communication, and it can be expected that the highly flexible transmission system or network can be constructed by providing meats for multiplexing and demultiplexing the wavelength division multiplexed optical signals.
In the wavelength division multiplexing optical transmission system, the indispensable elements includes the stabilization of the oscillation wavelength of the light source, the highly reliable optical multiplexing device for multiplexing a plurality of optical signals with different wavelengths into the optical fiber at low loss, and the highly reliable optical demultiplexing device for separating the optical signal in each wavelength from the wavelength division multiplexed optical signals and leading it to the optical receiver device. Here, the functions of multiplexing and demultiplexing can be provided by a single device in a form of an optical multiplexing and demultiplexing device.
Moreover, in the high density wavelength division multiplexing communication, it is indispensable for the optical multiplexing and demultiplexing device to have the multiplexing/demultiplexing characteristic of high resolution. In particular, in a case of realizing the wavelength division multiplexing by using optical amplifiers, in order to set up as many wavelengths as possible within the limited bandwidth of the optical amplifiers, it is necessary to improve the resolution of the optical multiplexing and demultiplexing device further.
Among the conventionally available devices, the most promising one is the optical multiplexing and demultiplexing device of the type in which the optical signals diffracted at the angles in accordance with their wavelengths are passed through optical elements such as lenses such that the dispersion of wavelengths are converted into the dispersion of positions.
A well known example of this type of the optical multiplexing and demultiplexing device is the Littrow type optical multiplexing and demultiplexing device shown in FIG. 1, which comprises a reflective diffraction grating 45, a lens 46, titanium diffused lithium niobate (LiNbO.sub.3) waveguide array 47.
In this Littrow type optical multiplexing and demultiplexing device, the wavelength division multiplexed optical signals 48-1 emitted from the uppermost waveguide channel in the waveguide array 47 are collimated by the lens 46 and lead to the diffraction grating 45 at which the optical signals are diffracted by diffraction angles in accordance with their wavelengths. Then, these diffracted optical signals are passed through the lens 46 again such that the optical signals of different wavelengths are collimated to different waveguide channels in the waveguide array 47 and outputted as the wavelength division demultiplexed optical signals 48.
Here, the resolution of the optical multiplexing and demultiplexing device is determined by the the pitch of the diffraction grating 45, the focal length of the lens 46, and the waveguide channel spacing of the waveguide array. In a case of the optical multiplexing and demultiplexing device using the titanium diffused lithium niobate waveguide array, this resolution has been 2 nm at best and for this reason there has been a limit to a number of channels that can be the wavelength division multiplexed.
On the other hand, when the density of the wavelength division multiplexing is raised, it becomes necessary to stabilize the oscillation wavelength of the light source, and the conventional available wavelength stabilization technique has been that which stabilizes the wavelength to the comb shaped resonant wavelengths of the Fabry-Perot resonator. However, in this conventional wavelength stabilization technique, the device size becomes very large.
In addition, there has been a problem that the stabilized wavelength may not necessarily coincide with the transmission wavelength of the optical multiplexing and demultiplexing device.
Moreover, in a case of the long distance transmission using the optical amplifiers, there has been a further problem that the transmission distance is going to be limited by the irregularity of the noises or the gains of the optical amplifiers.
It is also possible to form the above described Littrow type optical multiplexing and demultiplexing device using an optical fiber array as shown in FIG. 2, which comprises an optical fiber array 74 formed by arraying a plurality of optical fibers 74.sub.B, a lens 71 for collimating wavelength division multiplexed optical signals 70 emitted from each channel of this optical fiber array 74, and a diffraction grating 72 for diffracting the collimated wavelength division multiplexed optical signals 70 at the diffraction angles corresponding to their wavelengths so as to generate the optical signals 73 which are separated in wavelengths. Then, these diffracted optical signals 73 are passed through the lens 71 again such that the optical signals of different wavelengths are collimated to different optical fibers in the optical fiber array 74.
Another well known example of this type of the optical multiplexing and demultiplexing device is the Czerny-Turner type optical multiplexing and demultiplexing device shown in FIG. 3, which comprises an optical fiber 74.sub.B, a first lens 71.sub.i for collimating the wavelength division multiplexed optical signals 70 emitted from the optical fiber 74.sub.B, a diffraction grating 72 for diffracting the collimated wavelength division multiplexed optical signals 70 at the diffraction angles corresponding to their wavelengths so as to generate the optical signals 73 which are separated in wavelengths, a second lens 71.sub.2 for collimating these diffracted optical signals 73 and leading the resulting wavelength division demultiplexed optical signals to optical fiber array 74.
Now, when the wavelength division multiplexed optical signals emitted from the optical fiber and collimated by the lens have the incident angle a at the diffraction grating, the wavelength division multiplexed, optical signals are diffracted at the diffraction angles .beta. corresponding to their wavelengths according to the the relationship of the following equation (1): EQU .LAMBDA..multidot.(sin.alpha.+sin.beta.)=.+-.m.multidot..lambda.(1)
where .LAMBDA. is the grating period, .lambda. is a wavelength of the optical signal, and m is an order of diffraction.
By differentiating this equation by the diffraction angle .beta., the dispersion relationship between the diffraction angle and the wavelength can be obtained as in the following equation (2). EQU d.lambda./d.beta.=.+-..lambda..multidot.cos.beta./m (2)
This equation (2) shows that the difference in the wavelengths appears as the difference in diffraction angles.
In a case of the Littrow type optical multiplexing and demultiplexing device of FIG. 2, the demultiplexing function can be explained as follows. The optical signals 73 diffracted (i.e., wavelength dispersed) at different angles for different wavelengths are entered into the lens 71 again, and propagated for the focal length f of the lens 71, and coupled together on the surface of the optical fiber array 74. Here, by the operation of the lens 71, the optical signals 73 subjected to the diffraction angle dispersion are subjected to the position dispersion .delta. given by the following equation (3). EQU .delta.=f.multidot.tan(d.beta.).apprxeq.f.multidot.d.beta. (3)
Thus, the optical signals separated in wavelengths are positionally dispersed and coupled at a surface of the optical fiber array, such that the different optical fibers are going to receive the optical signals in different wavelengths. The demultiplexing function in the Czerny-Turner type optical multiplexing and demultiplexing device can also be explained similarly in principle, except for the use of two lenses instead of just one.
Using the above equations (2) and (3), the dispersion relationship between the wavelength of the optical signal (optical wavelength) and the coupling position on the optical fiber array surface can be written as follows. EQU .delta.=f.multidot.m.multidot.d.lambda./(cos.beta..multidot..LAMBDA.)(4)
FIG. 4 shows the transmission loss spectra for the optical multiplexing and demultiplexing device. Here, in order for the optical multiplexing and demultiplexing device using the lens and the diffraction grating to be applicable to the optical signals which are wavelength division multiplexed in high density, the optical multiplexing and demultiplexing device is required to have a high-resolution in its multiplexing and demultiplexing characteristic. From the above equation (4), it can be seen that the improvement of the resolution can be achieved by using the lens with a longer focal length f and the diffraction grating with a shorter grating period .LAMBDA..
However, when the lens with a longer focal length f is used, there arises the problem that the size of the device becomes large. In addition, there is also a problem that the coupling loss of the optical fiber and the optical signal becomes large as the aberration due to the lens on the optical fiber array surface also becomes larger. Moreover, there is also a problem of the disadvantageous temperature stability.
On the other hand, wren the diffraction grating with a shorter grating period .LAMBDA. is used, there arises the problem that the polarization dependency of the diffraction efficiency at the diffraction grating becomes large. In this regard, this polarization dependency has not been a serious problem in the devices designed for the observation of the optical spectrum such as the spectroscope, because the diffracted lights are optically received only after the polarization state of the incoming lights is fixed to some specific state by using a polarizer.
However, in the optical communication system such as the wavelength division multiplexed transmission system designed for the long distance optical communication in particular, this problem of the polarization dependency is going to be a serious problem as it deteriorates the transmission and reception characteristic.
Another conventionally known scheme for realizing the high resolution is the double pass scheme which is used in the optical spectrum analyzer. This scheme operates as shown in FIG. 5, in which the optical signals obtained by wavelength division demultiplexing the wavelength division multiplexed optical signals 70 emitted from the optical fiber 74.sub.B by using the lens 76 and the diffraction grating 72 are reflected back and entered into the lens 76 and the diffraction grating 72 again by means of the roof mirror (a pair of reflection mirrors) 77, so as to achieve the high resolution. In FIG. 5, there is also provided a wave plate 78 between the lens 76 and the roof mirror 77.
However, unless the optical fiber with a large core diameter or a photo-diode with a large light receiving area is available for receiving the diffracted light 75, it is quite difficult to realize the optical multiplexing and demultiplexing device with a high resolution in this scheme, for the following reasons.
First of all, the light passes through the lens 76 four times between the Input and the output, so that a problem that the aberration due to the lens becomes large.
Secondly, by providing the roof mirror, the reflected light of the roof mirror is passed through the lens 76, so that the aperture of the lens 76 must be larger for a part of the roof mirror compared with a case of the Littrow type optical multiplexing and demultiplexing device. For this reason, there arises the problem that it is difficult to make the coupling with the single mode optical fiber at low loss.
Also, when the multi-mode optical fiber with a large core diameter is used at the receiving side and in addition the length of the optical fiber is to be extended for a considerable length, the modal noise generated within the optical fiber becomes unignorable, so that it is not suitable for the long distance communication.
Thus, the double pass scheme is suitable for the application aimed at the observation of the relative strength or the signal spectrum of the optical signals as in the optical spectrum analyzer, but it is not suitable for the application to the optical multiplexing and demultiplexing device in the wavelength division multiplexing optical communication.